Battery Pack, Method Of Manufacturing Battery Pack, And Method Of Controlling Battery Pack

ABSTRACT

A battery pack having a plurality of electrically connected unit cells and configured so that the cells degrade at similar rates is provided. The battery pack may comprise a first unit cell and a second unit cell, wherein a temperature of the second unit cell is lower than the first unit cell. A condition of the first cell, such as states of charge or an open circuit voltage is set so that the condition of the first unit cell is less than a corresponding condition of the second unit cell. The unit cells may be thin battery cells stacked in a thickness direction of the thin battery cells, and the first unit cell may be located on an inner side of the second unit cell as viewed in a stacked direction. A temperature detecting unit may detect a temperature of each of the first and second unit cells and a charge control unit may be configured to control charging of the plurality of unit cells according to the temperatures of the first and second unit cells. With this configuration, even when the temperatures of the unit cells are not uniform, the rates of deterioration of the unit cells are equalized as much as possible.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to JapanesePatent Application No. 2006-062690 filed on Mar. 8, 2006. The entiredisclosure of Japanese Patent Application No. 2006-062690 is herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a battery pack having a plurality ofunit cells electrically interconnected, a method of manufacturing thebattery pack, and a method of controlling the battery pack.

2. Description of the Related Art

A battery pack having a structure in which a plurality of thin batterycells (unit cells) are stacked together and interconnected has beenknown as disclosed in Japanese Patent Application Laid-Open No.2003-346748 (Patent Document 1).

The battery pack described in the Patent Document 1 is structured suchthat a plurality of thin battery cells stacked together is contained ina box. The thin battery cells in the box are controlled so as to haveuniform values of voltage.

When a specific thin battery cell among the plurality deteriorates to agreater degree than the other thin battery cells, the deteriorated thinbattery cell must be replaced with a new one. To replace such a specificdeteriorated thin battery cell, substantial effort is required to takethe battery pack from the box and to disassemble it. Also, it is noteconomical to replace the whole battery pack when only one of theplurality of thin battery cells has deteriorated. For this reason, it isdesirable to use thin battery cells that deteriorate at similar rates.

SUMMARY OF THE INVENTION

The present invention has been proposed in light of the above-describedcircumstances. An object of the invention is to provide a battery packhaving a plurality of battery cells that deteriorate at similar rates, amethod of manufacturing the battery pack, and a method of controllingthe battery pack.

According to one aspect of the present invention, a battery pack havinga plurality of electrically connected unit cells is provided. Thebattery pack may comprise a first unit cell and a second unit cell,wherein a temperature of the second unit cell is lower than the firstunit cell. A condition of the first cell, such as states of charge or anopen circuit voltage is set so that the condition of the first unit cellis less than a corresponding condition of the second unit cell. The unitcells may be thin battery cells stacked in a thickness direction of thethin battery cells, and the first unit cell may be located on an innerside of the second unit cell as viewed in a stacked direction. Atemperature detecting unit may detect a temperature of each of the firstand second unit cells and a charge control unit may be configured tocontrol charging of the plurality of unit cells according to thetemperatures of the first and second unit cells detected by thetemperature detecting unit such that a condition such as states ofcharge or open circuit voltage of the first unit cell is less than acorresponding condition of the second unit cell having temperatureslower than those of the first unit cell.

According to another aspect of the present invention, a method ofmanufacturing a battery pack containing a plurality of flat batterycells stacked together and electrically connected is provided,comprising: charging first flat battery cell to a condition such as astate of charge or an open-circuit voltage; charging a second flatbattery cell located on the outer side of the first flat battery cell toa corresponding condition which is higher than the first condition, and;laminating and electrically connecting in series the first and secondflat battery cells.

According to still another aspect of the present invention, a method ofcontrolling a battery pack containing a plurality of electricallyconnected unit cells is provided, comprising: detecting a temperature ofeach of the unit cells; and controlling a condition such as states ofcharge of the unit cells or open circuit voltages of the unit cellsaccording to the detected temperatures of the unit cells. States ofcharge (SOC) or open-circuit voltages of first unit cells are set to belower than states of charge (SOC) or open-circuit voltages of secondunit cells. The second unit cells have temperatures lower than those ofthe first unit cells.

In one example, a battery pack according to the invention comprises aplurality of unit cells electrically interconnected. The states ofcharge (SOC) or open-circuit voltages of a first unit cell is set to belower than states of charge (SOC) or open-circuit voltages of a secondunit cell. The second unit cell have temperatures lower than those ofthe first unit cell. With this configuration, the rates of deteriorationof a plurality of thin battery cells can be equalized as much aspossible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are respectively a top view, a side view and a circuitdiagram, each showing a configuration of a battery pack to be controlledaccording to embodiments of the present invention;

FIG. 2 is a graph showing a relationship of an increasing rate ofinternal-resistance of a thin battery cell with respect to temperature;

FIG. 3 is a graph showing relationships between a state of charge (SOC)of a thin battery cell and an increasing rate of internal-resistance ofthe same;

FIG. 4 is a graph showing variations of the increasing rate ofinternal-resistance of a battery pack and a comparison with respect totime of use;

FIG. 5 is a block diagram showing a configuration for controllingvoltages of thin battery cells according to detected temperatures of thecells; and

FIG. 6 is a flowchart showing procedural steps of a process forcontrolling states of charge of thin battery cells.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will be described with reference to theaccompanying drawings.

In one example, a battery pack may be used as a power source forenergizing auxiliary devices of automobiles, such as a starter motor andheadlamps. The battery pack is located in, for example, an enginecompartment or a luggage compartment (trunk) of a vehicle, and used in atemperature range from normal temperature (atmospheric temperature) toabout 60° C.

The battery pack has four thin battery cells (unit cells) connected inseries as shown in FIG. 1. The thin battery 1 is a lithium ion batteryproducing a voltage of about 4.2 V in the full state-of-charge.Accordingly, the battery pack produces a voltage of about 16.8 V in thefull state-of-charge.

FIG. 1A is a top view showing the thin battery 1 (unit cell) comprisinga battery pack. A lamination of a positive electrode plate and anegative electrode plate is placed in an enclosing body, which is formedby shaping the covering film (such a lamination film) 1 a like a bag.Battery terminals 1 b and 1 c, which are connected to the cellcomponent, extend outside the enclosing body. It is assumed that thosethin battery cells, which form the battery pack, have the sameconstruction as thin battery 1.

As shown in the side view of FIG. 1B, the battery pack is constructedsuch that four sheets of thin battery cells 1A, 1B, 1C, and 1D (to becollectively referred to as a “thin battery 1”) are stacked, and thethin battery cells 1A and 1D (second unit cells) are located in theoutermost position as viewed from the side (cross-section view in thefigure) while the thin battery cells 1B and 1C (first unit cells) arelocated in the innermost position as viewed in the same direction.

The battery terminals 3, 4, 5, 6, 7, and 8 of the thin battery cells 1A,1B, 1C, and 1D are connected so as to form an electrical seriesconnection of those thin battery cells as shown in FIG. 1C. The batteryterminals of the thin battery cells 1A and 1D are connected to anelectrical apparatus (not shown) and supply electric power to theapparatus.

When the battery pack thus constructed supplies electric power to theelectric apparatus, the thin battery cells 1A, 1B, 1C, and 1D of thebattery pack discharge and supply the electric power. If equivalentcurrents flow through the stacked thin battery cells 1A, 1B, 1C, and 1D,the amount of heat generated by those battery cells are substantiallyequal to one another. In this case, temperature of the thin batterycells 1B and 1C, which are located on the inner side, among the thinbattery cells 1A, 1B, 1C, and 1D forming the battery pack, is higherthan that of the thin battery cells 1A and 1D on the outer sides.

FIG. 2 represents a variation of an increasing rate ofinternal-resistance with respect to the temperature at which the thinbattery cell is stored when the thin battery cells were stored for aboutsix months with a state of charge (SOC) of 50%. As seen from FIG. 2, asthe temperature rises, the increasing rate of internal-resistance (%)(i.e., battery deterioration rate) increases.

FIG. 3 represents a variation of the increasing rate ofinternal-resistance with respect to a state of charge of the thinbattery 1 when it is stored. The thin battery 1 was stored for about sixmonths at different temperatures (25° C., 45° C. and 55° C.). As seenfrom FIG. 3, as the SOC (%) of the thin battery 1 increases, theincreasing rate of internal-resistance (%) (i.e., battery deteriorationrate) increases.

The term “state of charge (SOC)” generally means a rate of remainingelectric energy (remaining capacity) to a storage electric energy(capacitance) when the battery is fully charged, and in thespecification, it will be referred to as a state of charge or SOC. Theterm “increasing rate of internal-resistance” means a rate of change ofthe internal resistance of the current battery to that of a new battery,and it is expressed in terms of %.

Thin battery cells 1A and 1D are located on the outer sides of the thinbattery 1 and therefore the heat generated at the time of charging anddischarging easily dissipates. The SOC of the thin battery cells 1A and1D is selected to be higher than that of the thin battery cells 1B and1C. Thin battery cells 1B and 1C are located on the inner sides of thethin battery cells 1A and 1D and therefore heat generated at the time ofthe charging/discharging hardly dissipates. Thereby, a variation of therates of deterioration (increasing rate of internal-resistance) of thethin battery cells 1A, 1B, 1C, and 1D forming the battery pack can beminimized. It is known that a correlation is present between the SOC ofthe thin battery and an open-circuit voltage. Namely, as the SOCincreases, the open-circuit circuit voltage increases. Therefore, asshown in FIG. 1C, the open-circuit voltages of the thin battery cells 1Band 1C (of which temperature rises highest at the time ofcharging/discharging) may be set to be lower than those of the thinbattery cells 1A and 1D (of which temperature does not rise as highabove the temperature of the thin battery cells 1B and 1C at the time ofcharging/discharging).

The term “open-circuit voltage” generally means a voltage between theterminals of the battery (electromotive voltage of the battery itself)at no load, and is called “open voltage” or “no-load voltage”.

In other words, the states of charge of the thin battery cells 1B and 1C(of which temperature rises highest at the time of charging/discharging)are set to be lower than those of the thin battery cells 1A and 1D (ofwhich temperature does not rise as high above the temperature of thethin battery cells 1B and 1C at the time of charting/discharging).Alternatively, the open-circuit voltage of the thin battery cells 1A and1D (of which the temperature does not rise as high at the time ofcharging/discharging) is set to be higher than that of the thin batterycells 1B and 1C (of which temperature rises highest above thetemperature of the thin battery cells 1A and 1D at the time ofcharging/discharging).

The internal-resistance rate, as stated above, is a rate (%) of changeof the internal resistance of a battery after it has deteriorated fromthe internal resistance of a new battery when the internal resistance ofthe new battery is set at 1. The increasing rate of internal-resistanceis mathematically expressed by:((R₁−R_(o))/R_(o))×100where R_(o) denotes the internal resistance of a new battery; and R₁denotes the internal resistance of the battery after it is deteriorated.

In the example shown in FIG. 1C, an average open-circuit voltage of thethin battery cells 1A, 1B, 1C, and 1D is set at 4 V. The open-circuitvoltages of the thin battery cells 1B and 1C is set at 3.95 V. Thetemperature of these cells rises highest at the time ofcharging/discharging (higher than the temperature of the thin batterycells 1A and 1D at the time of charging/discharging). The open-circuitvoltage of the thin battery cells 1A and 1D is set at 4.05 V. Thetemperature of these cells does not rise as high at the time ofcharging/discharging (lower than the temperature of the thin batterycells 1B and 1C at the time of charging/discharging).

The following option is also possible. The average states of charge ofthe thin battery cells 1A, 1B, 1C, and 1D are set at 80%; the states ofcharge of the thin battery cells 1B and 1C (of which temperature riseshighest at the time of charging/discharging) are each set at 75%; andthe states of charge of the thin battery cells 1A and 1D (of whichtemperature does not rise as high at the time of charging/discharging)are each set at 85%. It is known that a correlation is generally presentbetween the open-circuit voltage and the SOC. As the state of charge ofthe thin battery 1 is increased, the open-circuit voltage becomes high.Conversely, as the open-circuit voltage is increased, the state ofcharge of the thin battery 1 becomes high. In the example mentionedabove, the open-circuit voltage is 4.05 V when the state of charge ofthe thin battery 1 is 85%, and the open-circuit voltage is 3.95 V whenthe state of charge is 75%.

A relationship between the increasing rate of internal-resistance (%) ofthe battery pack and the time-of-use (day) is shown in FIG. 4. Thisrelationship occurs in two cases. The first case is when theopen-circuit voltages or the states of charge of the thin battery cells1B and 1C (of which temperature rises highest at the time ofcharging/discharging) are lower than the thin battery cells 1A and 1D(of which temperature does not rise high are change) (one embodiment ofthe present invention). The other case is when the open-circuit voltagesor the states of charge of a plurality of thin batteries 1 are equal toone another (comparison). Current in battery 1 was kept constant whilemeasuring the variation of increasing rate of internal-resistance shownin FIG. 4. The currents of the thin batteries 1 were kept constant atthe time of charging/discharging by repeating a sequence of four stepsof (1) charging, (2) charging/discharging rest, (3) discharging, and (4)charging/discharging rest.

For the discharge conditions, the current value is 10CA (able tocompletely discharge the fully charged battery at a fixed current forsix minutes), and the discharge end voltage (voltage at the end ofdischarge) is 2.5 V. For the charge conditions, the current value is10CA (equal to the current value at the time of discharging), and thecharge end voltage (voltage at the end of charging) is 4.2 V. Thecharging/discharging rest is one minute. The internal resistance wasmeasured in a manner that voltage dropped when the thin battery 1 wasdischarged at a fixed current, and a DC resistance value was calculatedby applying the current value and the voltage value to Ohm's law.

As seen from FIG. 4, the increasing rate of internal-resistance (%) ofthe battery pack of the one embodiment of the present invention moregently varies than that of a battery pack (comparison) in which thestates of charge (open-circuit voltages) of all the thin batteries areequal to one another.

The reason why a difference occurs among the increasing rate ofinternal-resistance of the battery packs will be described. Thetemperature of the thin battery 1 rises when the battery pack is chargedand discharged. The heat dissipation from the thin battery cells 1A and1D which are located on the outer side as viewed in the stackingdirection (hereinafter referred to as the outer side thin battery cells1A and 1D) is higher than that of the thin battery cells 1B and 1C whichare located on the inner sides as viewed in the stacking direction(hereinafter referred to as the inner side thin battery cells 1B and1C). Accordingly, the temperature of the inner side thin battery cells1B and 1C is higher than that of the outer side thin battery cells 1Aand 1D. Particularly, when the states of charge or the open-circuitvoltages of the battery 1 are equal as in the case of the comparison,the internal resistance of the inner side thin battery cells 1B and 1Cis higher than that of the outer side thin battery cells 1A and 1D.Accordingly, in the case of the battery pack of the comparison, theservice life of the inner side thin battery cells 1B and 1C determinesthat of the battery pack per se.

In a case where the states of charge or the open-circuit voltages of theinner side thin battery cells 1B and 1C are lower than the states ofcharge or the open-circuit voltages of the outer side thin battery cells1A and 1D, as in the case of the present application, a differencebetween the increasing rate of internal-resistance of the outer sidethin battery cells 1A and 1D and that of the inner side thin batterycells 1B and 1C is reduced. This results in the service life of thebattery pack of the invention being longer than that of the comparison.

In the embodiment, the states of charge or the open-circuit voltages ofthe inner side thin battery cells 1B and 1C are lower than the states ofcharge or the open-circuit voltages of the outer side thin battery cells1A and 1D. Accordingly, the service life of the inner side thin batterycells 1B and 1C is close to that of the outer side thin battery cells 1Aand 1D. In the example mentioned above, when the difference between thestates of charge of the thin battery cells 1B and 1C and the states ofcharge of the outer side thin battery cells 1A and 1D is limited to bewithin 10% of the maximum capacity of each thin battery 1, the servicelife of the outer side thin battery cells 1A and 1D is substantiallyequal to that of the inner side thin battery cells 1B and 1C. Also whenthe difference between the open-circuit voltages of the thin batterycells 1B and 1C and the open-circuit voltages of the outer side thinbattery cells 1A and 1D is limited to be within 0.1 V, the service lifeof the outer side thin battery cells 1A and 1D is substantially equal tothat of the inner side thin battery cells 1B and 1C.

The difference between the states of charge or the open-circuit voltagesof the thin battery cells 1B and 1C and the states of charge or theopen-circuit voltages of the thin battery cells 1A and 1D is much largerthan the difference between the states of charge or the open-circuitvoltages as mentioned above. In this case, the increasing rate ofinternal-resistance (caused by the difference between the states ofcharge of the inner side thin battery cells 1B and 1C and the outer sidethin battery cells 1A and 1D) or the open-circuit voltage difference, islarger than the increasing rate of internal-resistance caused by thetemperature difference between the inner side thin battery cells 1B and1C and the outer side thin battery cells 1A and 1D. Further, theincreasing rate of internal-resistance of the outer side thin batterycells 1A and 1D exceeds that of the inner side thin battery cells 1B and1C. For this reason, it is preferable that the difference between thestates of charge of the thin battery cells 1B and 1C (i.e., the loweststates of charge of the thin batteries) and the states of charge of theouter side thin battery cells 1A and 1D (i.e., the lowest states ofcharge of the thin batteries) is limited to be within 10% of the maximumcapacity of each thin battery 1. Alternatively, it is preferable thatthe difference between the open-circuit voltages of the thin batterycells 1B and 1C (i.e., the lowest open-circuit voltages of the thinbatteries) and the open-circuit voltages of the outer side thin batterycells 1A and 1D (i.e., the highest open-circuit voltages of the thinbatteries) is limited to be within 0.1 V.

In another embodiment of the present invention, a control unit is usedto regulate discharge of the cells. In the battery pack with the thinbattery 1 electrically connected in series, the currents flowing throughthe battery 1 at the time of charging/discharging are substantiallyequal to one another. Before those thin battery cells are connected inseries to form a battery pack, those batteries may be programmed so asto have different states of charge (open-circuit voltage difference).The batteries thus programmed, when connected in series, are charged anddischarged while keeping the state-of-charge difference (open-circuitvoltage difference). Specifically, assume a case where the battery pack,which includes a plurality of electrically series-connected thinbatteries stacked together, is installed in a luggage compartment(trunk) and is free from influence by outside temperature. In this case,the battery is charged such that the states of charge (SOC) or theopen-circuit voltages of the inner side thin battery cells 1B and 1C(first battery cells) are higher than those of the outer side thinbattery cells 1A and 1D (second battery cells). Following this, thosethin battery cells are stacked and electrically connected in series toform a battery pack, whereby the service lives of the thin battery cellsare uniformized as much as possible.

In a case where the battery pack is installed in an engine compartmentof a vehicle, for example, the battery pack is thermally affected by theengine. As a result, temperatures of the inner side thin battery cellsdo not always increase the most at the time of charging/discharging ofthe battery pack. To avoid this, it is preferable that the temperaturesof the batteries are detected. The states of charge (SOC) or theopen-circuit voltages of the batteries are then controlled according tothe detected temperatures. The embodiment is arranged as shown in FIG.5. Temperatures of the thin battery cells 1A, 1B, 1C, and 1D aredetected by temperature detecting units (temperature control means) 3A,3B, 3C, and 3D, and the states of charge or the open-circuit voltages ofthe thin battery cells 1A, 1B, 1C, and 1D are controlled by astate-of-charge (SOC) control unit 6. The state-of-charge (SOC) controlunit 6 consists of a controller 5, and discharging circuits 4A, 4B, 4C,and 4D.

The temperature detecting units 3A, 3B, 3C, and 3D are thermal sensorsfor detecting and outputting temperatures of the thin battery cells 1A,1B, 1C, and 1D. The outputs from the temperature detecting units 3A, 3B,3C, and 3D are output to a controller 5 of the SOC control unit 6.

The SOC control unit 6 contains the controller 5 and dischargingcircuits 4A, 4B, 4C, and 4D.

The discharging circuits 4A, 4B, 4C, and 4D are each a series circuit ofa switch and a resistor, and those series circuits are connected acrossthe thin battery cells 1A, 1B, 1C, and 1D, respectively. In thedischarging circuits 4A, 4B, 4C, and 4D, the switches are turned onaccording to commands from the controller 5 to consume the electricpower from the thin battery cells 1A, 1B, 1C, and 1D to control the SOCsof the thin battery cells 1A, 1B, 1C, and 1D, respectively.

The controller 5 reads the temperatures of the thin battery cells 1A,1B, 1C, and 1D output from the temperature detecting units 3A, 3B, 3C,and 3D. Controller 5 then prepares commands on the basis of thetemperatures of the thin battery cells and sends them to the dischargingcircuits 4A, 4B, 4C, and 4D to thereby control the switches of thedischarging circuits.

Operations of the controller 5 will be described by using a flow chartof FIG. 6. The process flow chart starts at the same time as thecontroller 5 is supplied with electric power from a power source (notshown) to power-up the controller 5.

In the embodiment description to follow, it is assumed that the thinbattery cells have been charged to uniform states of charge (forexample, 85%) when the controller 5 is started.

In step S1, the controller reads temperatures of the thin battery cells1A, 1B, 1C, and 1D output from the temperature detecting units 3A, 3B,3C, and 3D.

In the next step S2, the controller computes the maximum and the minimumtemperatures of those detected from all the thin battery cells. In stepS3, the controller calculates a difference between the maximum andminimum temperatures to check whether or not a variation is presentamong those temperatures. If the difference between the maximum andminimum temperatures is a predetermined value or higher, the controllerdetermines that a variation is present. If it is lower than thepredetermined value, the controller determines that no variation ispresent. If no variation is present, the controller ends the process. Ifa variation is present, the controller advances to step S4.

In the next step S4, the controller calculates the temperaturedifference between each thin battery cell and the minimum temperature.The controller sends timing signals based on the calculated temperaturedifferences to the discharging circuits 4A, 4B, 4C, and 4D, and turns onthe switches to discharge the electric power of the thin battery cells1A, 1B, 1C, and 1D. For example, the timing signals of the switches areadjusted to lower the state of charge by 1%, or to decrease thetemperature by 1° C. relative to the minimum temperature. This isequivalent to an open-circuit voltage reduction of 10 mV with respect to1° C. of the temperature difference relative to the minimum temperature.For example, in a case where the thin battery cell has a temperaturethat is 10° C. higher than the minimum temperature, the state of chargeof the thin battery cell is reduced to 10% (0.1 V of the open-circuitvoltage) below the state of charge of the thin battery cells having theminimum temperature.

In this case, as described above, it is preferable that the differencebetween the state of charge of the thin battery cells with the higheststate of charge and that of the thin battery cells with the lowest stateof charge is limited to be within 10%. Alternatively, it is preferablethat the difference between the open-circuit voltage of the thin batterycell having the lowest open-circuit voltage and that of the thin batterycells having the highest open-circuit voltage is limited to be within0.1 V.

For the thin battery cell with high temperature and a large increasingrate of internal-resistance, the SOC of the thin battery cell isreduced. Therefore, the increasing rate of internal-resistance owing tothe SOC is made small, whereby the rates of deterioration of the thinbattery cells 1A, 1B, 1C, and 1D are close to equal value. Accordingly,in the battery pack, the thin battery cells 1A, 1B, 1C, and 1D areadjusted to deteriorate at similar rates as much as possible. Theservice lives of the thin battery cells 1A, 1B, 1C, and 1D forming thebattery pack are substantially equalized. Therefore, there is no need todisassemble the battery and replace only the thin battery cell that hasdeteriorated more than the others with a new one. This leads to areduction of battery management cost.

It should be understood that the present invention is not limited to theembodiments mentioned above, but may be variously modified, altered andchanged within true spirits of the invention.

A stack of thin battery cells (unit cells) is used for the battery packin the embodiments mentioned above. In a case where the battery pack ofthe invention is used as a high-power battery pack, such as a powersource for a motor, or as a driving power source of a vehicle, unitbattery packs composed of a plurality of thin battery cells are formed,and those unit batteries are layered together to form a battery pack.Also in this case, temperatures of some unit battery packs rise high andtemperatures of some unit battery packs do not rise high. Accordingly,the service lives of those unit battery packs may be equalized bycontrolling the states of charge or the open-circuit voltages in themanner as described above. Specifically, the states of charge or theopen-circuit voltages of the high-temperature unit batteries are set tobe lower than those of the low-temperature unit battery packs, wherebythe state of charge or the open-circuit voltage is controlled for eachunit battery pack to equalize the service lives of the unit batterypacks.

The battery pack containing a stack of four thin battery cells has beendescribed in the embodiments. However, the number of thin battery cellsis not limited to four, but may be six or eight since the inventioninvolves the technical idea that as the temperature increases, the SOCor the open-circuit voltage decreases. Also, various embodimentsdescribed herein refer to “thin” battery cells, however the invention isnot necessarily limited to cells that are thin.

1. A battery pack having a plurality of electrically connected unitcells comprising: a first unit cell; and a second unit cell, wherein atemperature of the second unit cell is lower than the first unit cell,wherein a condition selected from the group consisting of states ofcharge and the open circuit voltage of the first unit cell is less thana corresponding condition of the second unit cell.
 2. The battery packaccording to claim 1, wherein the unit cells are thin battery cellsstacked in a thickness direction of the thin battery cells, and thefirst unit cell is located on an inner side of the second unit cell asviewed in a stacked direction.
 3. The battery pack according to claim 1,further comprising: a temperature detecting unit which detects atemperature of each of the first and second unit cells; and a chargecontrol unit configured to control charging of the plurality of unitcells according to the temperatures of the first and second unit cellsdetected by the temperature detecting unit such that a conditionselected from the group consisting of states of charge and the opencircuit voltage of the first unit cell is less than a correspondingcondition of the second unit cell having temperatures lower than thoseof the first unit cell.
 4. The battery pack according to claim 2,wherein a difference between a state of charge of a unit cell having thehighest state of charge among the plurality of unit cells and a state ofcharge of a unit cell having the lowest state of charge is 10% or less.5. The battery pack according to claim 2, wherein a difference betweenan open circuit voltage of a unit cell having the highest open circuitvoltage among the plurality of unit cells and an open circuit voltage ofa unit cell having the lowest open voltage is 0.1 V or less.
 6. A methodof manufacturing a battery pack containing a stack of a plurality ofunit cells that are electrically connected, comprising: charging a firstunit cell of the plurality of unit cells to a first condition selectedfrom a group consisting of a state of charge and an open circuitvoltage; charging a second unit cell of the plurality of unit cells to asecond condition selected from a group consisting of a state of chargeand open circuit voltage, the second condition being higher than thefirst condition; and stacking and electrically connecting in series thefirst and second unit cell such that the second unit cell is located onan outer sides of the first unit cell.
 7. A method of controlling abattery pack containing a plurality of unit cells electricallyconnected, comprising: detecting a temperature of each of a first andsecond unit cells; and controlling charging of the plurality of unitcells according to the detected temperatures of the first and secondunit cells such that a condition selected from a group consisting ofstates of charge and open circuit voltages is less for the first unitcell of the plurality of unit cells than a corresponding condition forthe second unit cell of the plurality of unit cells, where the secondunit cell have temperatures lower than temperatures of the first unitcell.
 8. A battery pack comprising: a plurality of unit cellselectrically interconnected to form a battery; temperature detectingmeans for detecting a temperature of each of a first and second unitcells; and a state-of-charge control means for controlling charging ofthe plurality of unit cells according to the temperatures of the firstand second unit cells detected by the temperature detecting means suchthat a condition selected from the group consisting of states of chargeand open circuit voltages of the first unit cell among the plurality ofunit cells is less than a corresponding condition of the second unitcell having temperatures lower than those of the first unit cell.